A multiple-bodied gas turbine engine comprises assemblies rotating independently of one another usually about one and the same axis. For example, a double-bodied engine comprises two assemblies, one called high pressure and the other low pressure. The high pressure body consists of a compressor and a turbine mounted on one and the same shaft. The high pressure compressor supplies the combustion chamber with air which itself delivers the combustion gases to the high pressure turbine. The low pressure body comprises a low pressure turbine receiving, through a channel called the transition channel and where necessary a distributor, the gases that have undergone a first expansion in the high pressure turbine.
One of the means of increasing the output of the low pressure turbine consists in reducing the aerodynamic load via an increase in the average radius of the latter. The radius of the high pressure turbine remaining unchanged, it follows that the geometry of the transition channel between the high pressure, HP, turbine and the low pressure BP, turbine is therefore to be adapted between its section for the inlet of the gases originating from the high pressure turbine and its outlet section emerging into the distributor for supplying the low pressure turbine. For aero engines, because of space and weight constraints, it is not opportune to lengthen the transition channel; it follows that the walls of the latter must have steep slopes and arrange a considerable diffusion. A limit is however imposed by the quality of flow that is to be retained at the walls; the thickening and even the detachment of the boundary layer must be avoided.
If the limits of slope and diffusion in the swan neck formed by the transition channel are exceeded, detachments of the boundary layer occur that are an unfavorable factor for the performance of the turbine. That would cancel out the gain provided by the increase in the average radius of the low pressure turbine.
To remedy this problem, a solution consists in re-energizing the boundary layer at the walls in order to prevent detachments of the boundary layer, by injecting a flow of fluid into the boundary layer.
Such a solution therefore allows the adoption of a transition channel from the HP turbine to the BP turbine:                with a steep slope in order to increase the average radius of the turbine and hence the output,        with high diffusion in order to reduce the losses generated by the distributor of the low pressure turbine and hence increase the output of the BP turbine.        
This solution is appropriate for any transition channel between two turbine sections, not only between the HP section and the BP section immediately downstream.